Method for establishing a communication link between subscriber stations of a switching system which comprises two communication networks

ABSTRACT

In a switching system, the subscriber stations of which are linked to a switching centers of a first communication network (NET 1 ), which are connected to each other through a second communication network (NET 2 ), such as the internet, by means of transition devices (MG 1  to MGn), with which separate control devices (MGC 1 , MGC 2 ) are associated to control the connection traffic as a function of control and/or signaling information, a communication connection is established to a subscriber station to be called (SUB 2 ), the availability of which has changed from one switching center (SW 2 ) to another switching center (SWn), by first setting up a signaling connection to the switching center (SW2) at which the subscriber station to be called (SUB 2 ) was hitherto available, whereupon a communication connection is established using the control and/or signaling information specifying the changed availability of the subscriber station to be called (SUB 2 ′) directly to the other switching center (SWn) at which the subscriber station to be called is now available, via the transition device (MG 2 ) connecting this switching center (SWn) to the second communication network (NET 2 ).

LIST OF REFERENCE CHARACTERS

[0001] List of reference characters IP network Internet protocol network MG1, MG2, MGn Transition devices MGC1, MGC2 Control devices NET1, NET2 Communication networks SUB1, SUB2, SUB2′ Subscriber stations SW1, SW2, SWn Switching centers SWC1, SWC2, SWCn Control devices TR Transition area

[0002] The invention relates to a method for establishing a communication connection between calling subscriber stations and subscriber stations to be called, which are linked to switching centers of a first communication network, in particular a time-multiplex switching network, the switching centers of which are connected to each other by means of transition devices through a second communication network, in particular a packet switching network, such as an IP network, whereby separate control devices are associated with the transition devices and control connection traffic via the transition devices assigned respectively to them as a function of the control and/or signaling information relating to the respective communication connection, which is transmitted during the course of signaling connections.

[0003] An IP network here refers to a switching and transmission network, in which communication signals are switched or transmitted on the basis of an internet protocol.

[0004] Switching network configurations of the type mentioned above will in future supplement or even supersede conventional communication networks, such as time-multiplex switching networks. The respective communication signals to be transmitted are converted in the transition devices between the switching centers of the first communication network and the second communication network according to the respective network conditions. If the first communication network is a time-multiplex switching network, such as for example the communication network deployed commercially under the name EWSD, and if the second communication network is for example a packet switching network, such as an IP network, it is necessary to effect the conversion from time-multiplex signals to packet signals and vice-versa from packet signals to time-multiplex signals in the transition devices between the switching centers of the first communication network and the second communication network respectively. This means that in the event of a communication connection between a calling subscriber station linked to a switching center of the first communication network and a subscriber station to be called also linked to a switching center of the first communication network, in particular another switching center of the relevant first communication network, a dual signal conversion is required based on the signal conversions considered above. This requires a relatively high conversion capacity and therefore computer capacity in the transition devices provided between the relevant switching centers of the first communication network and the second communication network, said transition devices generally being computer-controlled, in order to achieve the fastest possible conversions of the communication signals. Such fast conversions of communication signals are necessary so that the delays in the forwarding of communication signals resulting from the relevant conversions are not perceived as disruptive and detrimental to the quality of the respective communication connection.

[0005] In addition to the problem highlighted above, a further, in some cases more serious, problem also occurs in the event of call forwarding or call redirection in respect of a subscriber station to be called. With such call redirection a communication connection is first set up in the conventional manner to the switching center, at which the subscriber station to be called was hitherto available, and a redirection communication connection is then set up from this switching center to the switching center at which the relevant subscriber station to be called is now available. However, given the switching system configuration considered above, this means that in addition to the conversion processes considered above, further conversion processes also have to be implemented in respect of the communication signals to be transmitted in each instance between the calling subscriber station and the subscriber station to be called. However this is associated with marked delay times and therefore also a deterioration in quality during transmission of the respective communication signals between the calling subscriber station and the subscriber station to be called.

[0006] These delay times result primarily from packet assembly and disassembly processes due to the transitions between the said switching networks. Given a typical packet assembly or disassembly time of between 10 and 30 ms and a delay threshold value of around 100 ms due to the sensitivity of human hearing in respect of sound delays, an n-fold conversion of communication signals, by which the resulting overall delay time due to conversion exceeds the said threshold value, is no longer perceived as tolerable.

[0007] With regard to the delay threshold value mentioned above, it should also be taken into account that sections subject to a high level of delay may be included during the course of the respective communication connection, such as one or a plurality of mobile radio links or a satellite link.

[0008] The object of the invention is therefore to highlight a way in which, with a method of the type mentioned above, communication signals can be produced in a relatively simple manner via the second communication network between a calling subscriber station and a subscriber station to be called in the event of a call redirection on the direct route between the switching center of the first communication network, to which the calling subscriber station is linked, and the switching center of the first communication network, at which the subscriber station to be called is now available, whereby the problems addressed above are avoided, primarily with regard to the delay times mentioned.

[0009] The object highlighted above is achieved with a method of the type mentioned above according to the invention by establishing a communication connection to a subscriber station to be called, the availability of which has changed from one switching center to another switching center of the first communication network, in that during the course of a signaling connection established in the first place to the switching center at which the subscriber station to be called was hitherto available, it is established that this subscriber station is now available at a different switching center of the first communication network and that the communication connection to this other switching center is established directly via the transition device connecting this switching center to the second communication network using control and/or signaling information specifying the changed availability of the subscriber station to be called.

[0010] The invention has the advantage that in the event of a call redirection to a subscriber station to be called, the availability of which has changed from one switching center of the first communication network to another switching center of the first communication network, a communication connection can be established in practically the same way as a regular connection between a calling subscriber station linked to a switching center of the first communication network and a subscriber station to be called linked to another switching center of the first communication network. This means that in the event of call forwarding to a subscriber station to be called, the communication signals are only subject to the same number of conversion processes between the first communication network and the second communication network as in the case of a regular connection between two subscriber stations. The present invention thereby eliminates the problems that can arise as a result of the conventional call forwarding considered above. Primarily however the delay times addressed above can be reduced to a level that is no longer disruptive during voice signal transmission.

[0011] The said signaling connection between the switching center to which the calling subscriber station is linked, and the switching center at which the subscriber station to be called was hitherto available, is expediently set up via control devices, which on the one hand are connected to the relevant switching centers and the transition devices connected to these and which on the other hand are connected to each other. This advantageously allows signaling connections between calling subscriber stations and subscriber stations to be called to be established in a particularly flexible manner within the overall switching system. Existing standard devices, which are deployed for setting up communication connections in the respective switching network, can thereby be used in the individual switching networks.

[0012] A particularly efficient procedure in respect of the establishment of signaling and communication connections also results in that during the course of a signaling connection to the subscriber station to be called, the availability of which has changed from one switching center to another switching center of the first communication network, control and/or signaling information comprising the call numbers of the respective calling subscriber stations and subscriber stations to be called is stored in the control devices included in this signaling connection, the control device, which is connected to the switching center at which the subscriber station to be called was hitherto available receives control and/or signaling information supplied by the relevant switching center about the switching center at which the relevant subscriber station to be called is now available, and this control device identifies the existence of a call redirection situation from a comparison of the said control or signaling information supplied to it with the control and/or signaling information stored in it and thereupon uses the control or signaling information now available to it to effect the establishment of a communication connection between the second communication network and the switching center of the first communication network at which the subscriber station to be called is now available, via the transition device to which the relevant switching center of the first communication network and the second communication network are connected. This has the advantage that a relatively simple and reliable signaling procedure can be used before the communication connection is established between the calling subscriber station and the subscriber station to be called, the availability of which has changed from one switching center to another switching center of the first communication network. Existing standard devices, which are deployed for setting up communication connections in the respective switching network, can be used in the individual switching networks for this signaling procedure too.

[0013] The respective signaling connection between the respective calling subscriber station and the subscriber station to be called in each instance is expediently processed in a corresponding manner to the respective communication connection between the relevant subscriber stations. This has the advantage that, when it has been identified that there is a call redirection situation in respect of the subscriber station to be called, a signaling connection no longer has to be routed via the switching center of the first communication network at which the subscriber station to be called was hitherto available, thereby alleviating the signal processing load.

[0014] The control or signaling information stored in the said control devices of the transition devices involved in the respective connection is preferably deleted after termination of the communication connection between the subscriber stations connected to each other in each instance. This ensures that no unnecessary information or data remains in the control devices after termination of the respective communication connection.

[0015] The invention is described in more detail below with reference to a drawing.

[0016] The drawing shows an example of a switching system, comprising two different communication networks NET1 and NET2, which are connected to each other via a transition area TR.

[0017] Associated with the switching system shown in the drawing is a first communication system NET1, which may for example be a time-multiplex switching system, such as the EWSD system in commercial use. This first communication network NET1 has a plurality of switching centers SW1, SW2 to SWn, which are equipped with their own control devices SWC1, SWC2 to SWCn (only shown diagrammatically here). These control devices SWC1 to SWCn are used both to select the respective switching center as a function of control and/or signaling information, in particular dialing information, from subscriber stations which are linked to the respective switching center, and to receive and forward such control and/or signaling information from or to the transition area TR shown in the drawing. Linked to the switching centers SW1 to SWn of the first communication network NET1 is a plurality of subscriber stations, which may include telephone subscriber terminals, fax machines, data terminals, PCs or other communication terminals. In the drawing only one such subscriber station SUB1 is shown in respect of the switching center SW1. Subscriber stations SUB2 and SUB2′ are also shown in the drawing in respect of the switching centers SW2 and SWn, whereby the subscriber station SUB2 is shown with broken lines. It should be pointed out here that the subscriber station SUB2 was hitherto available at the switching center SW2 of the first communication network NET1 but is now available as the subscriber station SUB2′ at the switching center SWn of the first communication network NET1.

[0018] The second communication network NET2, which may for example be a packet switching network, in which communication signals are transmitted as ATM signals or as IP signals, as for example in the internet and which may therefore be formed by an IP network, is only shown diagrammatically in the drawing.

[0019] As shown in the drawing, the switching centers SW1 to SWn of the first communication network NET1 are connected to the second communication network NET2 via the transition area TR mentioned above. Associated with this transition area TR are individual or separate transition devices MG1, MG2 to MGn, via which the switching centers SW1, SW2 to SWn of the first communication network NET1 are connected to the second communication network NET2. Also associated with the relevant transition area TR are control devices, such as MGC1 and MGC2, which are assigned to the transition devices MG1 or MG2, MGn mentioned just above and the switching centers SW1 or SW2, SWn of the first communication network NET1 and which are also connected to each other. These control devices, of which the drawing only shows the two control devices MGC1 and MGC2, are used to set up signaling connections between calling subscriber stations and subscriber stations to be called, the communication connections of which are then set up via the associated switching centers of the first communication network NET1, the transition device MG1 to MGn of the transition area TR and the second communication network NET2. The method stages thereby implemented are described in more detail below in relation to a call redirection situation in respect of a subscriber station to be called.

[0020] According to the invention a communication connection between a calling subscriber station, such as the subscriber station SUB1 shown in the drawing, which is linked to the switching center SW1 of the first communication network NET1, and a subscriber station to be called, which was hitherto available as subscriber station SUB2 at the switching center SW2 of the first communication network NET1, but which is now available at another switching center, namely the switching center SWn of the first communication network NET1 and which is referred to there as SUB2′, is established after a signaling connection has first been set up. With regard to the subscriber station to be called, it should be noted that the subscriber station SUB2′ should still be available at the call number which was dialed by the calling subscriber station SUB1 to establish a communication connection to the relevant subscriber station to be called.

[0021] The above-mentioned signaling connection is first set up from the calling subscriber station SUB1 via the switching center SW1 or via the control device SWC1 associated with this, then via the control device MGC1 of the transition area TR connected to this switching center SW1 and from the control device MGC1 to the control device MGC2, which is associated with the switching center SW2, at which the subscriber station SUB2 to be called was hitherto available. The second communication network NET2 is not included in this signaling connection.

[0022] During the course of the respective signaling connection control or signaling information comprising the call number of the respective calling subscriber station and the subscribing station to be called in each instance is stored in the control devices MGC1, MGC2 of the transition system. During the course of the signaling connection considered above, control or signaling information is sent from the control device MGC2 to the switching center SW2 of the first communication network NET1, at which the subscriber station SUB2 to be called was hitherto available. This subscriber station SUB2 to be called has left behind in the switching center SW2 its call redirection address or new address, i.e. the call number, at which the relevant subscriber station SUB2 to be called is now available, in the present case at another switching center of the first communication network NET1. In the example shown this is the switching center SWn of the first communication network NET1, in which the relevant subscriber station to be called is now available as SUB2′.

[0023] The control device MGC2 thereupon receives control or signaling information back from the switching center SW2, said information comprising the call number of the calling subscriber station (SUB1), the call number of the subscriber station to be called (SUB2) and the call redirection number (SUB2′) of the relevant subscriber to be called. The control device MGC2 compares this information with the previously mentioned information stored in it relating to the calling subscriber station and the subscriber station to be called. From this comparison the control device MGC2 identifies that there is a call redirection situation, in other words that the required subscriber station SUB2 to be called is no longer available at the switching center SW2. The control device MGC2 also identifies from the relevant comparison where the required subscriber station to be called is now available. It then initiates, where necessary together with the above-mentioned control device MGC1, the setting up of a communication connection between the calling subscriber station SUB1 and the subscriber station SUB2′ via the switching center SW1 of the first communication network, the transition device MG1 of the transition area TR, the second communication network NET2, the transition device MGn of the transition area TR and the switching center SWn, as the switching center at which the subscriber station SUB2 to be called’ is now available.

[0024] The procedure described above ensures that when a call redirection situation exists in respect of a subscriber station to be called, only two switching centers of the first communication network NET1 and therefore only two transition devices, such as the transition devices MG1 and MGn in the example described, for a connection via the second communication network NET2, are included in the communication connection to be established between the calling subscriber station and the subscriber station to be called. Once different transmission procedures and formats have been established for communication signals in both communication networks NET1 and NET2, as mentioned above, only a dual communication signal conversion is required even in the event of the call redirection situation described. Even in this case the quality of the signal transmission is the same as in the case of a regular connection between a subscriber station linked to one switching center of the first communication network NET1 and a subscriber station to be called linked to another switching center of the same communication network NET1.

[0025] The signaling connection between the respective calling subscriber station and the subscriber station to be called in each instance can be processed in a corresponding manner to the communication connection between the relevant subscriber stations. This means, with regard to the exemplary embodiment according to the invention described above, that the signaling connection between the calling subscriber station SUB1 and the switching center SW2, at which the subscriber station SUB2 to be called was hitherto available, is in future no longer forwarded via this switching center SW2 after identification of the call redirection situation in respect of this subscriber station to be called but is sent directly from the control device MGC2 to the switching center SWn or received by said switching center, at which the subscriber station SUB2′ to be called is now available.

[0026] In order to alleviate the load on the individual control devices of the switching system shown after termination of the respectively established connections, the control or signaling information stored in the said control devices of the transition device involved in the respective connection is deleted after termination of the communication connection between the respectively connected subscriber stations. This ensures that only information about currently existing connections is contained in the relevant control devices.

[0027] In conclusion, it should also be pointed out that with the exemplary embodiment shown in the drawing the control device MGC2 is shown as a control device which is associated with both the switching centers SW2 and SWn of the first communication network NET1 and also the transition devices MG2 and MGn of the transition system TR, via which the relevant switching centers SW2, SWn are connected to the second communication network NET2. It may however also be clear that every switching center of the first communication network NET1 may be assigned a corresponding control device of the control area TR, via which the relevant switching center is connected to the second communication network NET2. In this case the control and/or signaling information is transmitted correspondingly between the individual control devices as in the case of transmission between the control devices MGC1 and MGC2, so that the establishment of the communication connection required in each instance between the calling subscriber station and the subscriber station to be called operates practically as described above. 

1. A method for establishing a communication connection between calling subscriber stations and subscriber stations to be called, comprising: linking the subscriber stations to switching centers of a first communication network; connecting the switching centers to each other by transition devices through a second communication network; assigning the transition devices separate control devices, which control connection traffic via the transition devices assigned respectively thereto; and storing call redirection to another subscriber station at another switching center, for a subscriber station to be called at one switching center, wherein during a signaling connection to the subscriber station to be called a first control and/or signaling, which comprises call numbers of the calling subscriber stations and the subscriber stations to be called in the control devices, which are included in the signaling connection, the control device receives a second control and/or signaling information element supplied by the switching center, which is connected to the control device and at which the subscriber station to be called was available, via the switching center at which a relevant subscriber station to be called is now available, the control device identifies existence of a call redirection situation from a comparison of the first and second control or signaling information elements, and the communication connection is set up to the other switching center from the control device using the second control and/or signaling information element directly via the transition device, which connects the switching center to the second communication network.
 2. The method according to claim 1, wherein the signaling connection between the switching center to which the calling subscriber station is linked, and the switching center at which the subscriber station to be called was available, is effected via control devices, which are either connected to the relevant switching centers and the transition devices connected thereto or which are connected to each other.
 3. The method according to claim 1, wherein the signaling connection between the calling subscriber station and the subscriber station to be called is processed in a corresponding manner to the communication connection between the subscriber stations.
 4. The method according to claim 1, wherein the control or signaling information elements stored in the control devices of the transition devices involved in the connection are deleted after termination of the communication connection between the connected subscriber stations. 